In operator

With "in," a built-in Python operator, we search strings and collections. Often these searches are the most efficient. The code is clear and readable and short.

The for-loop also uses an "in" keyword, but this one has a different meaning. It is just part of the for-loop, part of the enumeration expression.

In, string

This operator can search for individual characters within a string. The quotes used don't matter—we can use single or double quotes here.

value = "abc"

# This letter is found.
if "c" in value:
    print(True)

# This letter is not found.
if "x" not in value:
    print(True)
True
True

List

With a list, the "in" operator matches an existing element. If no matching element is found, "in" returns false and "not in" returns true. More than one value cannot be tested.

colors = ["blue", "green"]

# Test for blue string.
if "blue" in colors:
    print("BLUE FOUND")
BLUE FOUND

In, substrings

We can also search for substrings (or "slices") within strings. The entire substring specified must be found in the correct order. Incomplete matches return False.

value = "cat and dog"

# This substring is in the value.
if "and" in value:
    print(1)

# This one is not.
if "and cat" not in value:
    print(2)
1
2

Returns true, false

Most examples show the "in" operator inside an if-statement. This makes sense. But "in" is just part of an expression that returns True or False—a boolean.

elements = [10, 20, 30]

# This evaluates to true.
result = 20 in elements
print(result)

# False.
result = 40 in elements
print(result)
True
False

Dictionary

The "in" operator on dictionaries matches just keys. It does not consider the values. And we cannot pass it a tuple (pair) containing both: this does not work.

lookup = {"snake" : "green", "bird" : "blue"}

# Keys are searched with "in":
if "bird" in lookup:
    print(1)

# But values are not searched:
if "blue" not in lookup:
    print(2)
1
2

Tuple

This operator works well on tuples, with any number of items. We do not need a separate tuple variable for an if-statement—we can test a tuple specified directly within an if.

value = "cat"

# See if the value exists in the tuple.
if value in ("dog", "bird", "cat"):
    print(True)
True

String in, find benchmark

Usually, "in" is faster than alternative methods. As a Python developer, we should use "in" when possible as it is clear, easy to read, and fast.

import time

value = "characters"

print(time.time())

# Version 1: use "not in."
for i in range(0, 1000000):
    if "t" not in value:
        raise Exception()

print(time.time())

# Version 2: use find method.
for i in range(0, 1000000):
    if value.find("t") == -1:
        raise Exception()

print(time.time())
1406834210.134138
1406834210.259137  in   = 0.125 s
1406834210.696638  find = 0.437 s

Index, list

Often with lists, we need to locate a specific element and its index. The "in" operator does not work here. We can use "index" or a simple for-loop to do the search.

String find

We can use find and rfind to search for a string from the left or right side. This returns an index. With index() and rindex, an error is returned if no match is located.

In searches for a match within a string or collection. It is often the fastest option for search. Its syntax is clear and easy to read.